Chip electronic component

ABSTRACT

A chip electronic component including a ceramic element and terminal electrodes with metal coating thereon formed on the surface of the ceramic element. A glass layer is formed on a part of the surface of the ceramic element where the terminal electrodes are not formed. A glass material for the glass layer contains at least two species of alkali metal elements selected from Li, Na and K, and the total amount of the alkali metal elements is greater than or equal to 20 atomic percent of the total amount of elements except oxygen contained in the glass material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a chip electronic componentincluding a ceramic element and terminal electrodes having a metalcoating disposed on the ceramic element.

[0003] 2. Description of the Related Art

[0004] In recent years, needs for surface mountable electroniccomponents have been increasing and therefore needs for chip electroniccomponents have also been increasing. For example, a monolithic PTCthermistor is manufactured according to the following procedure: ceramicgreen sheets containing BaTiO₃ and conductive paste containingconductive powder such as Ni are alternately laminated, and sinteredintegrally to form a ceramic element. Then, terminal electrodescontaining Ag or the like are each formed on the end faces of theceramic element by a printing process.

[0005] A chip electronic component, manufactured according to the aboveprocedure, is normally soldered when it is mounted on a substrate.During the soldering step, terminal electrodes of the component are insome cases partly melted into the solder (this phenomenon is so-called“soldering erosion”) when the soldering temperature is higher than themelting point of the terminal electrodes or when the soldering time isextremely long. In order to prevent soldering erosion, the followingmethod has been employed: metal coatings containing Ni or the like areformed on the corresponding terminal electrodes by an electroplatingprocess before the soldering step.

[0006] However, when a ceramic element has a small sintered density,even if it is insulative, a plating solution permeates into the ceramicelement and deteriorates its properties. Furthermore, when terminalelectrodes sintered onto a semi-conductive ceramic element are subjectedto electroplating, there is a problem that metal coatings are alsoformed on portions of the element on which the terminal electrodes arenot disposed.

[0007] In order to solve the above problems, the following method hasbeen proposed: a ceramic element is soaked in a sodium-silicate, inwhich the atomic ratio of sodium to silicon is 0.6, to form a glasslayer on the ceramic element (refer to Japanese Unexamined PatentApplication Publication No. 2002-43167, pp., for example). Formation ofsuch an insulative glass layer on the ceramic element prevents metalcoatings from being formed on it. Furthermore, it has been thought that,by use of glass powder inclusive of a large amount of one of the alkalimetal elements, the difference in shrinkage between the ceramic layerand the glass layer can be reduced, thereby preventing cracks in theglass layer, because a melting point of glass is lower than usual.

[0008] However, a chip electronic component having a glass layer thereoncontaining a large quantity of one of the alkali metal elements hasproved to drop extremely in the dielectric strength according to theenergization test.

SUMMARY OF THE INVENTION

[0009] Accordingly, an object of the present invention is to provide achip electronic component where a sufficient dielectric strength issecured, and at the same time, a plating solution is prevented frompermeation into a ceramic element due to generation of cracks in a glasslayer when a glass layer is formed on the surface of a ceramic element.

[0010] In order to solve the above problems, a first preferredembodiment of the chip electronic component of the present inventionincludes a ceramic element and terminal electrodes having a metalcoating thereon disposed on the surface of the ceramic element. A glasslayer is disposed on at least portions of the ceramic element surface onwhich the terminal electrodes are not formed. The glass layer is made ofa glass material containing at least two species of alkali metalelements selected from Li, Na and K, and the atomic total amount of thealkali metal elements is greater than or equal to 20 atomic percent ofthe atomic total amount of elements, except oxygen.

[0011] In this chip electronic component, formation of cracks in theglass layer and deterioration in its dielectric strength can beprevented. More specifically, when a large amount of alkali metalelements are contained in a glass material to form a glass layer, alkalimetal elements are ionized and therefore ionic conduction takes place,whereby an electric current is allowed to flow on the surface of thechip electronic component and the dielectric strength is seriouslydeteriorated. Therefore, the inventors noticed that, from the fact thation conduction takes place via positions exclusively occupied by alkaliions, by using two or more species of alkali ions to occupy two or moredifferent positions in advance, the migration of the alkali ions isprevented. According to this configuration, even if a glass material toform a glass layer contains a large amount of alkali metal elements, thedielectric strength of a chip electronic component can be prevented frombeing deteriorated. Furthermore, the amount of the alkali elementscontained in a glass material can be increased compared to the prior artand therefore the melting point of the glass layer can be lowered, whichcan prevent cracks in a glass layer more efficiently.

[0012] In the above chip electronic component, as a second preferredembodiment of the present invention, the above at least two species ofalkali metal elements preferably include at least Li and K. When acombination of Li and K is employed as the two or more alkali metalelements, ionic conduction can be more securely prevented because of alarge difference between the ionic radii of a Li ion and a K ion.Furthermore, this case of chip electronic component, which can bemanufactured at low cost without deterioration in its properties, issuitable for mass-production.

[0013] In a third preferred embodiment of a chip electronic component ofthe present invention, among alkali metal elements contained in a glasslayer, the atomic ratio of two species of alkali metal elements having ahighest ranking much preferably falls in the range from 2:8 to 8:2. Bycontrol of the ratio of the alkali metal elements contained in the glasslayer within this range, the above ionic conduction can be moreefficiently prevented and a remarkable effect of suppression ofdeterioration in the dielectric strength can be brought out.

[0014] According to a fourth preferred embodiment of the presentinvention, the above ceramic element preferably contains asemi-conductive ceramic material. This case is more useful because metalcoatings, which are readily formed on a ceramic element when the ceramicelement is made of a semi-conductive ceramic material, can be preventedfrom being formed thereon due to the configurations according to thefirst or second preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic sectional view showing an embodiment of achip electronic component according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A chip electronic component of the present invention will bedescribed in detail with reference to the accompanying drawing.

[0017]FIG. 1 is a schematic sectional view showing an embodiment of thechip electronic component of the present invention. The chip electroniccomponent 1 of the present invention includes a ceramic layer 2 andinternal electrodes 3, to which terminal electrodes 5 electricallyconnected. A glass layer 6 is disposed on the surface of a ceramicelement 4. Ni coating 7 and Sn coating 8 are disposed on the surface ofeach terminal electrode 5.

[0018] Alkali metal elements contained in a glass material to form theglass layer 6 are, specifically, Li having an ionic radius of 0.068 nm,Na having an ionic radius of 0.097 nm, and K having an ionic radius of0.133 nm. Inclusion of two or more these alkali metal elements canprevent ionic conduction of the alkali metal elements. Among variouscombinations which may be used as the above combination of two or morealkali metal elements, a combination of Li and K is particularlypreferable. With this combination, ionic conduction can be more securelyprevented because of a large difference between the ionic radii of thetwo, and further, low-cost production is enabled without deteriorationin the electric properties of the chip electronic component.

[0019] Also, among the alkali metal elements contained in the glasslayer, the atomic ratio of the high-ranking two species of alkali metalelements whose containing amount is much preferably falls in the rangefrom 2:8 to 8:2. When the ratio is outside of the range, theeffectiveness, which can be obtained by giving two or more differentoccupied positions, is weakened.

[0020] Moreover, the atomic total amount of two or more species of thealkali metal elements preferably occupies greater than or equal to 20atomic percent of the atomic total amount of the elements, other thanoxygen, contained in the glass material. The use of a glass materialwith such composition prevents cracks from being formed in the glasslayer 6. When the atomic total amount of the alkali metal elements isless than 20 atomic percent of the atomic total amount of the elementsother than oxygen contained in the glass material, the effect that amelting point of glass is lowered drops off. Therefore, the differencein shrinkage between the glass layer 6 and the ceramic layer 2 cannot bedecreased and thus cracks are apt to be formed in the glass layer 6,which is not preferable. There is no upper limit to the atomic totalamount of the alkali metal elements, but the atomic total amount of thealkali metal elements is preferably less than or equal to 70 atomicpercent of the atomic total amount of the elements other than oxygencontained in the glass material. When the atomic total amount of thealkali metal elements is greater than 70 atomic percent, glass cannot beobtained, that is, the glass layer 6 is not formed on the ceramicelement 4.

[0021] The glass layer 6 can be also formed according to the procedurewhere glass powder together with an organic binder is dispersed andmixed into an organic solvent, and the mixture is applied as a glasspaste onto the end face of the ceramic element 4 and sintered.Alternatively, the glass layer 6 can be formed according to theprocedure where glass powder is dissolved in water to prepare a glassaqueous solution, in which the ceramic element 4 is soaked and thendried, though a method for forming the glass layer 6 is not limited tothe above.

[0022] As a ceramic material for the ceramic layer 2, any of asemi-conductive material, a dielectric material, a piezoelectricmaterial, a magnetic material and an insulative material can be used. Inparticular, a semi-conductive ceramic material provides greatadvantages. Such semi-conductive ceramic material includes transitionelement oxides with a negative temperature coefficient and zinc oxidewith varistor characteristics, as well as oxides such as barium titanatewith a positive temperature coefficient, for example, and is not limitedto these materials.

[0023] The terminal electrodes 5 are preferably formed with a powder ofan antioxidative noble metal such as Ag, Pd, Ag—Pd and Pt. A powder ofbase metal such as Ni and Cu may be used if the terminal electrodes 5are sintered in an antioxidative atmosphere. As a formation method ofthe terminal electrodes 5, the method in which the ceramic layer 2, Nielectrode paste for forming the internal electrodes 3, and conductivepaste for forming the terminal electrodes 5 are sintered at the sametime to form both end faces of the ceramic element 4 may be employed. Inaddition, a method in which conductive paste is applied onto thesintered ceramic element 4 and burned may also be employed.

[0024] In particular, ceramic green sheets for forming the ceramic layer2 and Ni electrode paste for forming the internal electrodes 3 arealternatively stacked, pressed, and cut into a predetermined size toform chips. After application of a semi-conductive paste for theterminal electrodes on their end faces, they are sintered in a reductiveatmosphere to form the terminal electrodes 5 on the end faces of theceramic element 4. By plating, Ni coating 7 and Sn coating 8 are formedon each terminal electrode 5, but materials for coating may be changeddepending on compatibility with the metal powder used for the terminalelectrode 5, and other than the above plating, solder plating can beused.

[0025] Below is a more specific explanation of a method formanufacturing a chip electronic component of the present invention,together with a method for manufacturing a chip PTC thermistor as anexample.

EXAMPLE

[0026] BaTiO₃, TiO₂, Sm₂O₃ and MnCO₃ were prepared as raw materials andthen compounded so that the following formula was satisfied:

(Ba_(0.997)Sm_(0.003))TiO₃+0.0005 Mn.

[0027] After pure water was added, the compounded raw material powderwas mixed and pulverized for 16 hours together with zirconia balls, andafter drying it was sintered temporarily at 1,200° C. for two hours andpulverized to form a temporarily-sintered powder. To thistemporarily-sintered powder, an organic binder, a dispersant and waterwere added and mixed for ten hours together with zirconia balls to forma ceramic slurry. This ceramic slurry was formed into sheets by a doctorblade process and dried to form ceramic green sheets. Next, Ni electrodepaste was applied onto a principal face of each ceramic green sheet by ascreen printing process so as to form a desired pattern. The ceramicgreen sheets were stacked so that the Ni electrode paste patterns wereplaced opposite to each other with a ceramic green sheet sandwichedtherebetween, and protective ceramic green sheets having no Ni electrodepaste were disposed at top and bottom thereof. The layers were pressedand cut into pieces with a size of 2.2 mm×1.4 mm×1.4 mm to form chips.Meanwhile, Ni electrode paste portions were exposed alternatively atboth end faces of each chip. The green chips were sintered at 1,200° C.for two hours in a reductive atmosphere in which the hydrogen content is3% of the nitrogen content, thereby obtaining a ceramic element 4including ceramic layers 2 and internal electrodes 3 alternativelylaminated.

[0028] Subsequently, aqueous solutions of alkali glass were prepared sothat the solutions have composition shown in samples 1 to 19 in Table 1.Here, a solid part in this aqueous solutions was set to 20 weight %. Inthese solutions the ceramic elements 4 prepared according to the aboveprocess were soaked and after drying, sintered at 600° C. in theatmosphere to form a glass sub-layer on each ceramic element 4. Thisprocedure was repeated twice to form a glass layer 6 consisting of twosub-layers. Conductive paste obtained from dispersion of Ag powder in anorganic vehicle was applied onto both end faces of the ceramic element 4having the glass layer 6 thereon. The conductive paste was sintered at700° C. in the atmosphere, and the glass layer existing between theceramic element 4 and the terminal electrodes 5 was dispersed into theterminal electrodes 5, so that conduction between the ceramic element 4and the terminal electrodes 5 was secured. In addition, this step ofsintering the conductive paste functions also as reoxidization of theceramic element 4. Finally, Ni coating 7 and Sn coating 8 weresequentially applied on the terminal electrodes 5 by an electroplatingprocess, to form a monolithic PTC thermistor 1. TABLE 1 AmountComposition (mol..) of First Second Al- Alkali Alkali kali Sample MetalMetal Composition ato- Number Element Element SiO₂ Formula mic *1 4 4 924(Li₂O)4(K₂O)92(SiO₂) 14.8 *2 5 5 90 5(Li₂O)5(K₂O)90(SiO₂) 18.2 3 6 6 886(Li₂O)6(K₂O)88(SiO₂) 21.4 4 8 8 84 8(Li₂O)8(K₂O)84(SiO₂) 27.6 5 10 1080 1O(Li₂O)10(K₂O)80(SiO₂) 33.3 6 15 15 70 15(Li₂O)15(K₂O)70(SiO₂) 46.2*7 20 0 80 20(Li₂O)80(SiO₂) 33.3 *8 20 0 80 20(K₂O)80(SiO₂) 33.3 *9 20 080 20(Na₂O)80(SiO₂) 33.3 *10 4 4 92 4(Na₂O)4(K₂O)92(SiO₂) 14.8 *11 5 590 5(Na₂O)5(K₂O)90(SiO₂) 18.2 12 6 6 88 6(Na₂O)6(K₂O)88(SiO₂) 21.4 13 1010 80 10(Na₂O)10(K₂O)80(SiO₂) 333 14 17 3 80 17(Li₂O)3(K₂O)80(SiO₂) 33.315 16 4 80 16(Li₂O)4(K₂O)80(SiO₂) 33,3 16 15 5 80 15(Li₂O)5(K₂O)80(SiO₂)33.3 17 5 15 80 5(Li₂O)15(K₂O)80(SiO₂) 33.3 18 4 16 804(Li₂O)16(K₂O)80(SiO₂) 33.3 19 3 17 80 3(Li₂O)17(K₂O)80(SiO₂) 33.3

[0029] With the above samples, characteristics and physicalities wereevaluated as to the following points. Evaluation results are shown inTable 2.

[0030] Presence or Absence of Cracks in Glass layer

[0031] The surface of the glass layer formed on the surface of eachceramic element of the monolithic PTC thermistors in samples 1 to 19 wasinspected by an optical microscope to check whether there were cracks inthe glass layer 6 or not.

[0032] Presence or Absence of Permeation of Plating Solution

[0033] The terminal electrodes 5 each having Ni coating 7 and Sn coating8 thereon were removed from the monolithic PTC thermistors in samples 1to 19. The ceramic element 4 with its terminal electrodes removed wasdissolved by acid into a solution, and Sn amount contained in thissolution was determined by ICP-AES analysis, a quantitative analysisbased on a difference between the light energy of Sn element and thelight energy of the other components.

[0034] Dielectric Strength

[0035] In order to inspect whether ionic conduction took place or not onthe surface of the ceramic element, a predetermined voltage was appliedto the monolithic PTC thermistors in samples 1 to 19 for three minutes.Dielectric strength, that is, magnitude of voltage when each monolithicPTC thermistor was destroyed by application of three minutes' voltagewas measured. In addition, obtained figures of dielectric strength wererounded off to the decimal point. TABLE 2 Permeation of Plating SolutionSample Cracks in Glass Sn Amount in Ceramic Dielectric Breakdown Numberlayer Element (ppm) Voltage (V) *1 Observed 65 6 *2 Observed 25 15 3 NotObserved 10 or less 23 4 Not Observed 10 or less 24 5 Not Observed 10 orless 23 6 Not Observed 10 or less 22 *7 Not Observed 10 or less 12 *8Not Observed 10 or less 13 *9 Not Observed 10 or less 12 *10 Observed 458 *11 Observed 20 13 12 Not Observed 10 or less 20 13 Not Observed 10 orless 20 14 Not Observed 10 or less 15 15 Not Observed 10 or less 22 16Not Observed 10 or less 22 17 Not Observed 10 or less 22 18 Not Observed10 or less 22 19 Not Observed 10 or less 16

[0036] As shown in Table 2, in samples 3-6 and 12-19 that are within thescope of the present invention,there are no cracks in the glass layer 6and permeation of plating solution is also acceptable at 10 ppm or less.Furthermore, in chip-type electronic components of which the made of theglass material in samples 3-6 and 12-19, the dielectric breakdownvoltage is about 1 5V or more, thereby having a sufficient dielectricstrength. In particular, in sample 3-6 and 15-18, where Li and K areused as a combination of two or more species of alkali metal elementsand its atomic ratio falls in the range from 2:8 to 8:2, the dielectricbreakdown voltage is excellent, about 22 V or more. In contrast, samples1, 2, 10 and 11 where the amount of alkali metal elements is less than20 atomic percent of the atomic total amount of elements except oxygencontained in the glass material, there are cracks in the glass layer 6without enough effect by alkali metal elements, and the plating solutionpermeated into the ceramic element 4 by 25 ppm or more. Furthermore, insamples 7-9 that do not contain at least two species of alkali metalelements, a sufficient dielectric strength is not obtained due to theoccurrence of ionic conduction, although there are no cracks in theglass layer 6 and permeation of the plating solution is adequatelyprevented. Also, in samples 14 and 19 where the atomic ratio of the twospecies of alkali metal elements having a highest ranking among thealkali metal elements contained in the glass layer, is outside of therange from 2:8 to 8:2, though they are covered by the present invention,the dielectric breakdown voltage is somewhat decreased.

[0037] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention is not limited by the specificdisclosure herein, but only by the appended claims.

What is claimed is:
 1. A chip electronic component comprising: a ceramicelement; terminal electrodes having a metal coating formed on a surfaceof the ceramic element; and a glass layer formed on at least a part of asurface of the ceramic element where the terminal electrodes are notformed, wherein the glass layer contains at least two species of alkalimetal elements selected from Li, Na and K, and the atomic total amountof the alkali metal elements is greater than or equal to 20 atomicpercent of the atomic total amount of elements except oxygen containedin the glass layer.
 2. A chip electronic component according to claim 1,wherein at least Li and K are included as the alkali metal elements. 3.A chip electronic component according to claim 1, wherein the atomicratio of the two species of alkali metal elements having a highestranking among the alkali metal elements contained in the glass layer arein a ratio from 2:8 to 8:2.
 4. A chip electronic component according toclaim 1, wherein the ceramic element contains a semi-conductive ceramicmaterial.
 5. A chip electronic component according to claim 2, whereinthe atomic ratio of the two species of alkali metal elements having ahighest ranking among the alkali metal elements contained in the glasslayer are in a ratio from 2:8 to 8:2.
 6. A chip electronic componentaccording to claim 2, wherein the ceramic element contains asemi-conductive ceramic material.
 7. A chip electronic componentaccording to claim 3, wherein the ceramic element contains asemi-conductive ceramic material.
 8. A chip electronic componentaccording to claim 5, wherein the ceramic element contains asemi-conductive ceramic material.